"Ken Moon" wrote in message
nk.net...
Bill,
See comments interspersed:
"Bill Stock" wrote in message
...
This is somewhat OT, but you appear to be the biggest suppository of
knowledge when it comes to NiChrome.

I want to build a Hot Wire to bend some Acrylic. The wire in question is
#22
and approximately 1 ohm per foot.
==============================
Is that at room temperature or at 400 degrees?

25°C.

Nichrome, as all metals, change resistance as temperature rises.
Therefore,
you'll have higher currents (Amps) at lower temps, so your supply needs to
have adequate reserve to heat the wire. As it reaches the higher temps,
the
current will self limit according to voltage available.
================================
Most of the temperature tables I've seen for NiChrome wire show the
relationship between Amps and Temperature. But shouldn't this really be
Watts? For example the tables show that I need about 3 Amps for the
400°F
that I require. The tables I've seen don't actually show temps that low.
But
assuming the tables are referring to 120V, that would mean 360 Watts or
15
Amps at 24 Volts? This can't be right?
================================
The normal Ohms law rules won't work here since the resistance is variable
as well as the Voltage. (Ohm's law is valid, just not as you'd normally
think of it in linear terms).
================================
The second problem is the power supply. Assuming I'm confused about the
Watts and 3 amps at 24 Volts will suffice. The average light dimmer will
only handle about 500 Watts, but if I short out (2 Ohms) the transformer
secondary I will fry the dimmer and/or transformer (DAMHIKT). I've seen
the
light bulb in series trick, but even a 200 Watt bulb on the Transformer
secondary would not give me enough Amps to reach my temperature.
Although
I
suppose a 200 Watt bulb on the Transformer primary would give me 8 Amps
at
24V?

There's obviously something I don't understand here. I would appreciate
it
if someone could lead me out of the darkness.
===================================
If you could find an old filament transformer like used in old tube type
radios, etc., it would probably get you into the desired range, but you'd
need to find out more about the cold to hot characteristics of the
Nichrome.


The adjustment factor is only 1.037 for the temp I want.